1. Field of the Invention
The present invention relates to an apparatus for fixing or locking a movable stage which is to be moved in the state floating above a stationary base or platform.
2. Description of the Prior Art
A movable stage or carriage locking apparatus which allows a movable stage to be moved in the state floating above a stationary base and subsequently to be fixed or locked at a predetermined position is known. Reference may be made to an article titled "A Precise Positioning Mechanism for Aerostatically Floatable Stage" contained in "Collection of Lectures at Congression of Japan Precision Machinery Association" No. 323, (1982, Spring) p. 207.
For having a better understanding of the present invention, description will first be made about a hitherto known apparatus. Referring to FIGS. 1 and 2, a table or stage 1 movable in a horizontal direction is combined with a plate-like or flat vacuum suction cap 2b provided with an air passage 2d connected to a pressurized air supply source 10 by means of an elastic member 2 provided with elastic connecting arms 2a so that the flat vacuum suction cap 2b is vertically movable and swingable relative to the stage 1. The vacuum suction plate or cap 2b is disposed in opposition to a stationary fixing base 5. For movement of the stage 1, the vacuum suction cap 2b is caused to move away from the stationary base 5 under resilience of the elastic connecting arms 2a. On the other hand, when the stage 1 is to be positioned fixedly, a conduit 6 connected to the air passage 2d is operatively connected to a vacuum source 8 under the control of an electrically controlled valve 7, as the result of which the air present under the vacuum suction cap 2b is discharged through the passage 2d, whereby the vacuum suction cap 2b is tightly locked to the stationary base 5 under action of the vacuum. In order to assure the movement of the vacuum suction cap or plate 2b in parallel with the stationary base or platform 5, a plurality of adjusting screws 11 arranged in through-holes 2e formed in the elastic member 2 and having respective internal threads are provided at appropriate positions. Through rotation of the adjusting screws 11 whose tips can bear against the flat top surface of the vacuum suction cap 2b, a gap 4 between the vacuum suction cap 2 and the stationary base 5 is adjusted in the unlocked state of the vacuum suction cap 2b. In practice, however, since a limitation is imposed on the pitch of the thread of the adjusting screw 11, difficulty will be encountered in finely adjusting the gap to be parallel between the vacuum suction cap 2b and the stationary base 5 on the order of microns in the unlocked state of the former. Further, in consideration of the fact that the adjustment of interest has to be performed by manipulating the adjusting screws 11 individually in the state in which the vacuum lock assembly including the elastic member 2, the vacuum suction cap 2b, the adjusting screws 11 and others are installed on the stationary base or platform 5, the adjusting operation is very troublesome. Besides, it is difficult to perform the adjustment with a high accuracy. It should further be mentioned that the vacuum lock assembly which is adapted to be employed in a machine requiring a higher positioning accuracy must be so constructed that the gap 4 between the vacuum suction plate or cap 2b and the stationary base 5 be decreased to a possible minimum in the unlocked state while assuring high parallelism therebetween. The hitherto known apparatus can not meet these requirements, not to speak of its low efficiency in operation and unsatisfactory accuracy of adjustment.